NASA's prolific Kepler space telescope, which has discovered more than half of all known planets beyond our solar system, just celebrated six years in space. The $600 million Kepler mission blasted off atop a United Launch Alliance Delta II rocket from Florida's Cape Canaveral Air Force Station on the night of March 6, 2009 (March 7 GMT). After a two-month commissioning phase, Kepler began searching for exoplanets, and began etching its name into the history books. Kepler finds alien worlds by watching for the tiny brightness dips they cause when they cross the face of their host stars from the spacecraft's perspective. (During its original mission, Kepler stared at more than 150,000 stars simultaneously.)

To date, the sun-orbiting spacecraft has discovered 1,019 exoplanets, with more than 3,100 additional "candidates" awaiting confirmation by follow-up observations or analysis. Mission scientists expect that around 90 percent of these potential planets will end up being the real deal. To put Kepler's tally into perspective: Scientists have discovered a total of about 1,800 alien planets.